Protective circuit for cathode ray tube



Nov. 26, 1963 Filed Feb. 17, 1960 2 sec 2 0 I) 2 4 sec INVENTOR PoulTheisen i' w w. BY 1, H

ATTORNEY 4 Sheets-Sheet 1 Nov. 26, 1963 v P. THEISEN 351125425PROTECTIVE CIRCUIT FOR CATHODE RAY TUBE Filed Feb. 17, 1960 4Sheets-Sheet 2 INVENTOR Poul Theisen BY 1' ,7 CM

ATTORNEY Nov. 26, 1963 P. THEISEN 3,112,425

PROTECTIVE CIRCUIT FOR CATHODE RAY TUBE Filed Feb. 17, 1960 4Sheets-Sheet s INVENTOR Poul Theisen ATTORNEY Nov. 26, 1963 P. THEISEN3,112,425

- PROTECTIVE CIRCUIT FOR CATHODE RAY TUBE Filed Feb. 17, 1960 4Sheets-Sheet 4 INVENTOR Poul Theisen ATTORNEY United States PatentOfltice Patented jd i ff i 1 3,112,425 PROTECTIVE CIRCUIT FOR CATHDDERAY TUBE Paul Theisen, Hannover, Germany, assignor to TelefunkenG.m.b.H., Berlin, Germany I Filed Feb. 17, 1960, Ser. No. 9,327 Claimsprior'ty, application Germany Feb. 19, 1959 Claims. (til. 31530) Thepresent invention relates to improvements in circuits for cathode raytubes where the tube has a fluorescent screen over which an electronbeam is deflected during operation, tubes of this type being use-dparticularly in television receivers and Oscilloscopes.

In general the highly sensitive screenof a cathode ray tube must beprotected from any continuous impingement of electrons onto the samespot or area; in other words, a cathode ray beam must be focused in sucha tube only when the beam deflecting device is also in operationsweeping the beam over the screen. When an electron beam impinges uponthe fluorescent layer of the video screen, such as the center thereof,even for only a short period of time, the fluorescent layer at the spotof impingement will be quickly burned.

It is known in the art to reduce automatically the intensity of acathode ray beam when the deflecting device becomes inoperative for anyreason.

Television receivers in which the line deflecting circuit is also usedfor producing the high accelerating voltage for the cathode ray tube, donot need such protection because when the line deflection circuit failsno high voltage will be produced for acceleration of the beam. However,it has been found that a protection device is necessary when thereceiver is turned off. This is due to the fact that the picture tubeaccelerating anode has a sizeable capacitance constituted by theconductive layer deposited inside and outside of the envelope, whichcapacitance is used as a filter for the rectifier delivering highvoltage peak pulses fromthe line deflection transformer during flyback.This capacitance usually amounts to 2,000 micromicrofarads. Upon cutolfof the receiver, the charge of this capacitance can be dissipated onlythrough the remaining cathode ray beam. This discharge is visible untilthe remaining charge is insutficient to maintain further current flow ofthe cathode ray beam, i.e., until the anode cannot draw any electronsfrom the rapidly cooling cathode. In case of minimum picture brightness,i.e., a weak beam during the final instant of operation, almost nocharge will flow from this picture tube capacitance after cutofi of thereceiver because the electron beam will rapidly become too weak tomaintain current flow. Only in case of maximum brightness during thefinal instant of operation will a considerable charge flow from theanode capacitance after cutoff of the receiver. Due to the fact that theglass wall of a picture tube has a high dielectric constant, theresidual charge thereon may remain even for days. Particularly, longafter the receiver has been turned ofl, there still is a high voltage of10 kilovolts or more present in the tube. If the tube cathode is capableof emitting electrons relatively long even after its heatercurrent hasbeen turned off, and if furthermore the control voltage for the cathoderay beam is reduced to zero relatively fast, an electron beam may againbegin to flow, discharging partially or completely the high potentialcharge on the tube. No deflection is present during this discharge andthus the electron beam impinges continuously for a considerable periodof time on the video screen thus damaging the fluorescent layer in thecenter thereof.

To avoid the last-mentioned effect, it is known to connect the grid ofthe cathode ray tube to a brightness con 'trol network having a longtime constant, the remaining control voltage suppressing partially theflow of cathode current for some time after cutoff of the receiver. Inthe case of rapid decay of the cathode voltage, .when the time constant,which is determined by the power supply to which the circuit is coupled,is low, there flows, after the cathode voltage has decayed, a currentwhich eliminates the negative space charge at the cathode and thusprevents a ray electron beam despite the residual charge of the highvoltage capacitance. However, this measure is no longer available if thecathode remains capable of electron emission for an appreciable timeafter receiver cutoff, and if in addition thereto, there is a straightelectrode system involved as is the case, for example, in defleetingtubes of In that case, the time constant element would have to be solarge as to result in a very slow-operating brightness adjustment duringoperation of the receiver, which is a highly undesirable feature. Thus,in this prior art arrangement having the usual highly emissive cathodeand a reasonably fast acting brightness control, the discharge of theanode capacitance through an undeflected electron beam after receivercutolf seems inevitable. It has been suggested to discharge the anodecapacitance by means of an additional contact associated with the on-oifswitch of the receiver and adapted to shunt the picture tube layersduring cutoff. This, however, would be inelfective if one turns thereceiver ofi merely by unplugging it from the power line.

It is an object of the present invention to provide a new and improvedcircuit for the cathode ray tube system of a high-frequency videoreceiver which new circuit provides complete anode capacitance dischargeimmediately after receiver cutoff, i.e., during the time interval inwhich the associated deflecting circuit is still operative.

It is another object of the invention to provide a new and improvedcircuit for discharging the anode capacitance in a cathode ray picturetube after cutoff of the receiver power supply by which this tube issupplied Without producing an undeflected cathode ray beam, whereby allthe deficiencies of the systems mentioned above are avoided.

It is a further object of the invention to discharge the cathode raytube capacitance through the electron beam during the time the positiveplate voltage is decaying in the other control circuits of the cathodray tube.

According to one aspect of the invention, in a preferred embodimentthereof, it is suggested to connect a control network to the controlgrid or other control element of a cathode ray tube, which controlnetwork maintains the brightness control substantially unchanged butautomatically varies the grid voltage for a short period of time afterreceiver cutolf in such a Way as to permit substantially maximum cathodebeam electron flow. In particular, the network is triggered by thereceiver cutoff and operates in dependence upon the decay of variousD.C. supply voltages thereafter. This nework may, for example, comprisean electronic switching arrangement which changes substantially the timeconstant of the brightness control network upon decay of the positivepower supply for this control network and the anode voltage.Alternatively, the operating point of the brightness control can beshifted in dependence uponthe decaying positive power supply with theaid of either non-linear circuit elements or the faster decayingnegative power supply or both.

Additional objects and advantages of the present invention will becomeapparent upon consideration of the following description when taken inconjunction with the accompanying drawings in which:

FIGURE 1 is a circuit diagram of the brightness control for a cathoderay tube having a video screen, and includes a network according to oneembodiment of the invention.

FIGURE 2 is a plot of the cathode ray tube grid voltaliases age againsttime if the network according to the invention were not used in thecircuit of FIGURE 1.

FIGURE 3 is a similar plot if the inventive arrangement is used.

FIGURE 4 is a plot of the cathode ray tube grid voltage if a controlnetwork having a large time constant is used according to one prior artdevice discussed above.

FIGURE 5 illustrates a practical example of a circuit diagram designedaccording to the principle employed and explained in connection withFIGURE 1.

FIGURE 6 is a circuit diagram of another embodiment of the inventionused in the brightness control network of a television receiver circuit.

FIGURES 6a and 6b are plots of decaying negative and positive powersupply voltages, respectively, as they ap pear in FIGURE 6.

FIGURE 7 is a modified form of the circuit of FIG- URE 6.

FIGURE 8 is a modification of the circuit diagram shown in FIGURE 7.

FIGURE 9 is a plot of the cathode ray tube grid control voltage asproduced by the circuit shown in FIGURE 8 after receiver cutoff.

FIGURE 10 is a circuit diagram of another embodiment of the invention.

FIGURE 11 is a modification of the circuit shown in FIGURE 10.

FIGURE 1 shows a cathode ray tube such as is used in televisionreceivers or oscilloscopes. The cathode 1 of tube 1 receives videosignals 3- including synch pulses 3 from a video amplifier 2. The videoamplifier is shown only schematically because it is of conventionaltype.

The high voltage power for the cathode ray tube 1 is supplied from aline deflection: transformer 5 having an oscillator 4 with electronicinput control 4' connected to its primary winding 5. The high voltagepeaks during retrace are used for powering the tube 1, and for thispurpose these peaks are fed through a diode 6 to a capacitor which isformed by two conductive coatings 7 and 8 on the glass wall of thetube 1. Coating 7 is deposited on the outside of tube 1 while coating 8is deposited on the inside. Coating 8 is connected electrically to theanode of tube 1 (not shown). The thus-formed capacitor is the storagecapacitor of the DC. accelerating power supply for the tube 1.

Grid 10 in tube 1 controls the brightness of the image produced by thecathode ray on the screen 9. Grid 10 is connected to a tap 11 of apotentiometer 12 which is connected between ground and a positive D-.C.voltage B+. The tap 11 is connected to grid 10 via two resistors 13 and14. The position of the sliding tap 11 determines the intensity of thebeam and therefore the brightness of the image on screen. 9. Negativeblanking pulses 16 are also supplied to grid 10 through a capacitor 15.These pulses bias the grid sufiiciently negative to block the cathoderay beam during retrace. The circuit as described thus far isconventional.

It has been found that with such a circuit, the screen 9* can be damagedwhen the receiver is turned off while the brightness control is in arelatively low position, i.e., when the cathode beam intensity was lowduring operation just before cutofl. In this case, the charge stored incapacitor 78 cannot be quickly discharged through the beam, as pointedout above. Thus, due to the fact that glass is a very good dielectricmaterial, this high voltage can remain for a very long time. This initself would not be a detriment if the cathode were not capable ofemission even if the other electrode voltages are already reduced tozero. However, the cathode is in fact capable of emission long afterreceiver cutoff and thus the charge on the capacitor will discharge inthe form of cathode ray beam current. This cathode ray beam will not bedeflected and thus will damage the center of the fluorescent layer ofthe image screen 9.

The following circuit arrangement according to the invention preventssuch damage. A capacitor 17 is connected between the grid 10 and anegative voltage produced additionally in the receiver. One can obtainsuch negative auxiliary voltage, for example, by an additional secondarywinding 5" on the line deflection transformer 5, which winding 5" isconnected to another diode 18 via a capacitor. The polarity of diode 18is selected so as to produce a DC. output voltage from the peak voltagesappearing across winding 5" during retrace. Diode 18 can be replaced byany known non-linear resistance element. The diode 18 and thetransformer secondary 5" may already be present in known televisionreceivers, for example, to derive a control signal from the deflectiontransformer to be used in the deflection control circuit. However, it isnot known to feed such a negative auxiliary voltage to the brightnesscontrol grid 10 of the video tube 1.

During operation of the video tube 1 and its control device anequilibrium is established across capacitor 17. The time constant of thebrightness control remains substantially unaffected by the chargedequilibrium of capacitor 17, because this capacitor can be selected tobe of relatively small value, for example, in the range of half amicrofarad. Upon cutting off the receiver, the capacitor 17 dischargeswith the result that two different discharge currents flow through thecapacitor 17 because capacitor 17 was charged on the upper plate by the13+ voltage and separately charged on the lower plate by the negativeauxiliary voltage from diode 18. In case the auxiliary voltage fed tocapacitor 17 via diode 18 were positive, these discharge currents wouldhave the same direction. However, due to the fact that this auxiliaryvoltage is negative, these discharge currents are in opposition. Uponproper selection of this auxiliary voltage one can in fact adjust thetotal discharge time of capacitor 17. If the current produced by theauxiliary voltage has the same order of magnitude as the other dischargecurrent, then the voltage at grid 10, as compared with ground, decayswith delay and the eflective capacitance of capacitor 17 is increased.It the auxiliary discharge current exceeds the opposite dischargecurrent, the grid voltage may even become positive for a short period.In other words, upon proper proportioning of the auxiliary voltagesource the grid voltage can be elevated for a short period of time to arelatively high positive voltage without the use of a mechanical switchbut by a similar electronic switching effect. The cutoff time constantof the decaying negative auxiliary voltage as shown in the drawing isgiven by the delay circuit elements associated with the transformer 5producing the auxiliary voltage. The time constant of the transformerdepends additionally on the decay time of the receiver power supplyproducing the 13+. In a given receiver with a given power supply source,this time constant of the negative auxiliary voltage is fixed andpredetermined. This is true also of the 13+ power supply and thebrightness control circuit.

FIGURE 2 shows the discharge characteristics, i.e., the decaying voltageof grid 10 if the capacitor 17 were directly connected to ground, andnot to diode 18.

FIGURE 3 shows the voltage characteristics if the same capacitor 17 isconnected to the negative potential, which is the salient feature of oneembodiment of the invention. As can be seen from the plot in FIGURE 3,the grid voltage increases positively to such a value, which in fact issufiiciently high to discharge completely the anode capacitance of thepicture tube in less than one second even if the brightness control wasadjusted for its lowest position corresponding to minimum brightness.During this time, the deflection electrodes in tube 1 are stilloperative and sweep the cathode ray over the screen 9. It will beobserved that the elements 4, 5 and 18 serve as a non-lineartransmission network for the decaying B+ which powers the oscillator 4.This non-linear network causes a reduction in the time constant of thedecay;

i.e., the negative voltage at the anode of diode 18 decays further thanthe B+ at the input of oscillator 4.

It has been found that a similar complete discharge could also beobtained if the capacitor 17 were connected directly to ground, as knownin the art, but this would require a capacitor whose capacitance isabout 2.5 microfarads' Even then, there would still be no positivevoltage peak at the grid 10 but the grid voltage would remain positivesufficiently long to discharge the anode capacitance. However, the timeconstant of such capacitor would be 2.5 seconds, which is undesirablebecause it renders the brightness control too slow, and the deflectionwill not operate long enough after receiver cutoff. This is shown inFIGURE 4, while when the curves of FIG- URES 2 and 3 were measured andplotted, the capacitor 17 was selected to have a capacitance of 0.47microfarad.

While FIGURE 1 illustrates only the principal circuit elements requiredto carry out the invention, FIGURE 5 is a complete schematic diagramthereof. Similar circuit elements are denoted with like referencenumerals, and the elements are further provided with circuit valueswhich have been found suitable when tested and used. To avoid brightnessjumps in case of power supply voltage variations in FIGURE 5, a resistor19 is connected .in series with capacitor 17, which resistor 19additionally serves as a substitute for resistor 14 of FIGURE 1 which isomitted in FIGURE 5. The peak of the voltage at grid will be somewhatreduced but this has no effect as far as the fast discharge of the anodeis concerned.

In FIGURE 6 the grid 10 of tube 1 is connected directly to a tap 11' ofa brigthness control potentiometer 12. This potentiometer is connectedbetween a positive voltage source B+ and the junction of two ohmic resistors 21 and 22 which are connected between a positive voltage B+which may be equal to the B+ feeding potentiometer 12' and a negativevoltage B- produced by the line deflection transformer during retrace.When the receiver is turned off, the positive supply voltage B+ decayswith a time constant determined by the power supply filter network ofthe receiver. The negative voltage B- also decays but with a differenttime constant which is determined by the deflection transformer and itsinput and output circuits. In general, the negative voltage decaysfaster than the positive voltages from the power supply with itscomparatively large filter capacitors, etc. Thus, the potential atjunction 20 having a fixed value 'during operation, for example zero,now shifts suddenly to a positive value after cutoff of the receiver.This in turn causes the voltage of tap 11' and of grid 10 to increase toa positive value whereby the cathode ray current of tube 1 is increasedand remains so until the high voltage of the anode capacitance is bledoff. The deflection circuits are still in operation and the increasedcathode ray sweeps over a substantial part of the video screen 9.

FIGURES 6a and 6b illustrate the decay time of the negative and of thepositive voltages, respectively, between which the resistors 21 and 22are connected.

FIGURE 7 is another embodiment of the invention. Here, the timeconstants of the positive and of the negative supply voltages are notespecially important any more. The positive voltage of B+ is fed tojunction 20' via a resistor 22', the negative voltage from negativeauxiliary voltage B supply source is also fed to the junction 20 via anonlinear resistor 23 which has an approximately constant value athigher currents. Junction 20' is connected to the potentiometer 12'as.was junction 20 in FIGURE 6. The decaying positive voltage B+ istransmitted to junction 20 in an undistorted configuration while thenon-linear resistor 23 changes the decaying negative. voltage to such anextent that the potential of junction 20' shifts to positive values.Here one can use positive and negative supply voltages of similar decaytime constants but still will obtain a positive shift of the gridvoltage of the cathode ray tube after receiver cutoff. If

the supply voltages are of unequal decay time constants, with thenegative supply decaying faster as shown in FIG- URES 6a and 6b, thenon-linear resistor 23 increases the effect and produces a higherpositive grid voltage after cutoff whereby the anode capacitance of thecathode ray tube is discharged even faster. The connection of junction26' to the cathode of a diode 24 having its anode connected to ground isoptional and serves to stabilize variations in the power supply for thereceiver which must not aifect the brightness of the television picture.Alternatively, diode 24 may be connected to a positive or a negativevoltage of such magnitude that diode 24 is conductive during operationof the receiver.

FIGURE 8 illustrates a modification of the circuit shown in FIGURE 7.The grid 10 of tube 1 is connected via a resistor 40 to a first junction41, which junction is connected to ground via a capacitor 42. Junction41 is further connected to a brightness control potentiometer 43, aswell as to a second junction 44 with a diode 45. Junction 44interconnects an ohmic resistor 46 and a glow discharge tube 47.Resistor 46 and glow discharge tube 47 are connected as a voltagedivider between a positive and a negative supply voltage. Junction 44 isfurthermore connected to ground via a capacitor 48.

During operation, diode 45 is blocked. Upon cutoff of the receiver, thenegative voltage decays faster than the positive voltage and with theadditional effect of the nonlinear resistance of the glow discharge tube47 the potential of junction 44 follows a curve plotted in FIGURE 9. Inthis embodiment, the adjustment of the network producing the positivegrid voltage after cutoff does not affect the brightness control 43which thus can be dimensioned within greater scope.

In FIGURE 10, the control grid 10 of cathode ray tube 1 is connected tothe tap 51 of a brightness control potentiometer 52. One terminal ofpotentiometer 52 is connected to a positive voltage, the other terminalis connected to the anode of a triode 53, the control grid of which isconnected to a junction 54 of a voltage divider 55, 56, which in turn isconnected between a positive voltage supply source and ground. Thejunction 54 is further connected to the positive voltage via a capacitor57.

The circuit as described thus far produces a control voltage at grid 10of cathode ray tube 1 which is determined by the voltage taken from thepotentiometer 52 and by the grid bias of triode 53 which has low ohmicresistance during operation of the receiver. Upon cutoff of the receiverthe decay of the positive voltage is transmitted to the grid of triode53 via the capacitor 57, whereby the triode immediately becomes a highresistance. The time constant for blocking triode 53 is very small ascompared with the time constants of the power supply decay. This controlaction in turn immediately renders the tap 51 positive as compared withits potential at the same position but prior to cutoff of the receiver.This potential increase at tap 51 even exceeds any voltage chargeproduced across potentiometer 52 and thus a very high positive voltagepeak appears at grid 10 of cathode ray tube 1.

For a further increase of this effect it can be of ad vantageadditionally to connect the anode of triode 53 to the positive voltagesupply via a high ohmic resistor 58. This increases the speed of thevoltage increase at tap 51 upon cutoff of triode 53.

FIGURE 11 illustrates a modification 0f the circuit shown in FIGURE 10.Elements 66, 67, 68, 69, and 70 and their mode of interconnectioncorrespond to elements 58, 53, 57, 55, and 56, respectively, in FIGURE10. The brightness control potentiometer 62 is connected between apositive voltage and ground and thereby it is partially connected inparallel to triode 67 which has a grounded cathode. Grid 10 of cathoderay tube 1 is connected to a tap 61 of potentiometer 62 via a resistor63. The junction 34 of the grid lead-in and resistor 7 63 is connectedto the anode of triode 67 via a resistor 65. The anode circuit of triode67 thus includes a voltage divider formed by resistors 63 and 65.

During operation, the grid 10 of tube 1 receives a stable bias voltagewhich is determined by the dimensioning of resistors 62, 63 and 65 andthe internal resistance of triode 67 at the given grid control voltagethereof. A variation of the position of tap 61 varies this bias voltagefor grid 10. Upon cutoif of the receiver, the positive supply voltagedecays according to a given function and a given time constant. Thevariation of this varying supply voltage is transmitted to the controlgrid of triode 67 via the capacitor 68. Thereby the triode 67immediately becomes substantially blocked and forms a high resistance.This in turn shifts the potential of junction 34 by a predeterminedamount which is larger than the positive voltage at any time aftercutoff of the receiver. Thus, the control grid 10 receives a highpositive control voltage increasing the cathode ray current for a shortperiod of time.

Triode 67 can, if desired, be replaced by a multigrid tube or any othercontrollable device which can be varied between high and low internalresistance.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

I claim:

1. In a control circuit for a cathode ray tube having a screen and atleast one control element for controlling the intensity of the cathoderay beam and having an accelerating anode capacitance charged to a highvoltage by a power source during operation, an improved circuit fordischarging said capacitance through the beam when the source is cutoff, said circuit comprising: control means connected to said controlelement for adjusting the operating bias level thereon, and voltagetranslating network means connected between said control element andsaid power source for producing a positive transient voltage on saidelement at a level increasing above said bias level during decay of thepower source when the latter is cut off, said network means including asecond power source having a decay function different from that of thefirst-mentioned power source and said transient voltage being derivedfrom the superposition of decay transients obtained from said powersources and serving as the control voltage.

2. In a control circuit for a cathode ray tube having a screen and atleast one control element for controlling the intensity of the cathoderay beam and having an accelerating anode capacitance charged duringoperation to a high voltage by a power source having a predetermineddecay function when cut off, an improved circuit for discharging saidcapacitance through the beam when the source is cut cfif, said circuitcomprising: control means connected to said control element foradjusting the operating bias level thereon, and non-linear network meansconnected between said control element and said power source fortranslating said decay function to a positive transient voltage andapplying it on said element to raise the voltage thereon above said biaslevel during decay of the power source when the latter is cut oif, saidnetwork means including a second power source having a decay functiondifferent from that of the first-mentioned power source and saidtransient voltage being derived from the superposition of decaytransients obtained from said power sources and serving as the controlvoltage.

3. In a control circuit for a cathode ray tube having a screen and atleast one control element for controlling the intensity of the cathoderay beam and having an accelerating anode capacitance charged duringoperation to a high positive voltage by a power source having, uponcut-off, a certain decay rate, an improved circuit for discharging saidcapacitance through the beam when the source is cut oif, said circuitcomprising: potentiometer means having a tap connected to said controlelement for adjusting the operating bias level therein, an auxiliarynegative power source having, upon cut-off, a decay rate which is fasterthan that of the positive power source, and means, connected betweensaid control element and said auxiliary negative power source forproducing a positive transient control voltage on said element at alevel increasing above said bias level during decay of the positivepower source when the sources are cut off.

4. In a circuit as set forth in claim 3, beam deflection means includinga deflection transformer, and said auxiliary source comprising a Windingon said transformer, and rectifier means connected with said winding anddelivering negative pulses during sweep return traces.

5. A control circuit for a cathode ray tube having a video screen, acontrol grid and an anode capacitance connected with a high voltagepower supply, said control circuit comprising: bias control meansconnected to said grid, a resistance voltage divider having a junctionbetween resistors, a positive voltage supply source having apredetermined decay time-constant when out off, a negative voltagesupply source having a cutoff time constant smaller than said firstmentioned time constant, said voltage divider being connected in seriesbetween said two voltage supply sources, and resistance means connectedbetween said junction and said grid.

6. In a control circuit as set forth in claim 5, said bias control meanscomprising a potentiometer connected to a positive source of voltage atone end and having a tap connected to said grid, said potentiometercomprising said resistance means and having its other end connected tosaid junction.

7. In a control circuit as set forth in claim 5, said voltage dividerincluding a non-linear resistance connected between said junction andsaid negative voltage source.

8. A control circuit as set forth in claim 7, said non-linear resistorbeing a glow discharge tube.

9. In a control circuit as set forth in claim 5, said last mentionedmeans including a unidirectional conductor element having a cathode andan anode, said anode being connected to said junction, said cathodebeing connected to the grid circuit.

10. In a control network as set forth in claim 5, a diode having acathode and an anode, said cathode being connected to said junction andsaid anode being connected to a constant potential biasing the diodeconductive to drive a current therethrough until said sources are .cutoff.

References Cited in the file of this patent UNITED STATES PATENTS2,638,562 Schipper et al May 12, 1953 2,714,177 Jones July 26-, 19552,743,380 Pratt Apr. 24, 1956 2,752,525 Montague June 26, 1956 FOREIGNPATENTS 762,440 Great Britain Nov. 28, 1956 213,501, Australia .d Mar.5, 1958

1. IN A CONTROL CIRCUIT FOR A CATHODE RAY TUBE HAVING A SCREEN AND ATLEAST ONE CONTROL ELEMENT FOR CONTROLLING THE INTENSITY OF THE CATHODERAY BEAM AND HAVING AN ACCELERATING ANODE CAPACITANCE CHARGED TO A HIGHVOLTAGE BY A POWER SOURCE DURING OPERATION, AN IMPROVED CIRCUIT FORDISCHARGING SAID CAPACITANCE THROUGH THE BEAM WHEN THE SOURCE IS CUTOFF, SAID CIRCUIT COMPRISING: CONTROL MEANS CONNECTED TO SAID CONTROLELEMENT FOR ADJUSTING THE OPERATING BIAS LEVEL THEREON, AND VOLTAGETRANSLATING NETWORK MEANS CONNECTED BETWEEN SAID CONTROL ELEMENT ANDSAID POWER SOURCE FOR PRODUCING A POSITIVE TRANSIENT